SR 1 and 2 Hay Point Fatigue Damage Structural Risk

Background SR 1 and 2 have been in operation for around 38 years They were commissioned in 1974 and have reclaimed 300 MT of coal

Structural Fatigue Structural fatigue arises when a cyclic stress state causes a progressive failure of a material In structural steelwork this typically occurs at welded joints or discontinuities

Structural Fatigue Design Standards • Current fatigue design codes have details with a reliability of 0.9773 or a 1 in 45 probability of cracking at the end of their design life. This compares to structural strength design which has a reliability of 0.999. Also the reliability decreases with age of the structure. • Satisfactory control of fatigue damage relies on adequate methods of fatigue crack detection and the ability to repair or replace the damaged component.

Structural Fatigue US Bridge Industry • Some precedent from Bridge Industry in the US to operate up to 16% probability of failure for structures which can’t readily be replaced. • Relies on thorough inspections

Fatigue Life Assessment of SR1 and 2 Fatigue assessment has been carried out on the machines which shows that they are at the end of their fatigue life Fatigue cracking is now occurring requiring repairs to be made and regular inspections are being carried out to try and detect cracking

Interim Life Extension Retrofit An Interim Life Extension Retrofit was proposed until SR1 & SR2 can be replaced Rebuild lower luffing lug assembly Replace counterweight pivot section Replace back section of boom Repairs to counterweight boom bracing connections Inspection and weld repairs SR1 and SR2 Risk Management

Results of Inspections SR2

Results of Inspections SR2

Results of Inspections SR2

Results of Inspections SR2

Fatigue Assessment SR2 Based on Citec data & Simulated loads More joints examined in detail Probability of Cracking calculated to BS 7608 Fracture mechanics crack growth curves developed

BS7608 Probability of Failure at 340MT Site Inspection Results Fatigue Assessment SR2 Location Weld Description BS7608 Probability of Failure at 340MT Site Inspection Results CW07 diaphragm plate to web plate, counterweight boom, RHS 6.8% Crack found CW09 web plate (22mm) to pivot box plate (51mm), counterweight boom, LHS 3.0% CW12a web plate (22mm) to pivot box plate (51mm), top position, counterweight boom, RHS 2.6% CW18a web plate to top flange of counterweight boom, RHS 18.1% CW35 gusset plate to top flange (along top flange), counterweight boom, LHS 27.2% CW54 gusset plate to bottom flange, counterweight boom, LHS 13.8% Cracks found

BS7608 Probability of Failure at 340MT Site Inspection Results Fatigue Assessment SR2 Location Weld Description BS7608 Probability of Failure at 340MT Site Inspection Results CW08 diaphragm plate to web plate, counterweight boom, LHS 19.5%   CW13 web plate (22mm) to pivot box plate (51mm), middle position, counterweight boom, RHS 3.7% CW17 top flange of drive support to top flange of counterweight boom, along top flange, LHS 5.1% CW18 web plate to top flange of counterweight boom, LHS 18.2% CW31 gusset plateto top flange (along diagonal brace), counterweight boom, LHS 23.5% CW32 gusset plate to top flange (along diagonal brace), counterweight boom, RHS 37.5% CW33 15.6% CW34 9.3% CW55b web stiffener to web plate (outside), counterweight boom, RHS 5.3%

BS7608 Probability of Failure at 340MT Site Inspection Results Fatigue Assessment SR2 Location Weld Description BS7608 Probability of Failure at 340MT Site Inspection Results CW04 web to flange, counterweight boom, RHS <0.1% Crack found CW05 web to flange, counterweight boom, LHS CW06a web plate (22mm) to pivot box plate (51mm), counterweight boom, RHS CW10 Internal horizontal stiffener to internal vertical stiffener of slew cone wall 0.90% CW11a 0.10% CW14 web plate (22mm) to pivot box plate (51mm), bottom position, counterweight boom, LHS CW15 web plate (22mm) to pivot box plate (51mm), top position, counterweight boom, LHS CW15a 0.70% CW15c horizontal stiffener on diaphragm plate to web plate, counterweight boom, RHS CW19a web stiffener plate to web (along the stiffener), counterweight boom, RHS CW37 gusset plate to top flange (along diagonal brace), counterweight boom, RHS 0.80% CW39 Cracks found CW51a gusset plate to bottom flange , counterweight boom, LHS 0.30% CW52 gusset plate to bottom flange of boom (along diagonal brace), counterweight boom, RHS

BS7608 Probability of Failure at 340MT Site Inspection Results Fatigue Assessment SR2 Location Weld Description BS7608 Probability of Failure at 340MT Site Inspection Results CR01-1 cone stiffener to top plate of cone 5.9% Cracks found CR01-2 48.0% CR01-3a Stiffener plate of lug to bottom flange of RHS girder 28.0%

BS7608 Probability of Failure at 340MT Site Inspection Results Fatigue Assessment SR2 Location Weld Description BS7608 Probability of Failure at 340MT Site Inspection Results BWB01 Boom wall to lug, luff end, bucketwheel boom, RHS 35.1% Crack previously found BWB02 Diaphragm plate to boom wall, luff end, bucketwheel boom, LHS 42.8% BWB05 Longitudinal stiffener to boom wall, bucketwheel boom, LHS 2.4%  

Results of Inspections SR1

Results of Inspections SR1

Results of Inspections SR1

Results of Inspections SR1

Results of Inspections SR1

Results of Inspections SR1

Results of Inspections SR1

Results of Inspections SR1

Results of Inspections SR1

Results of Inspections SR1

Results of Inspections SR1

Crack growth – fracture mechanics Crack growth will increase rapidly and become critical within a given period for the detail under consideration. Inspection intervals at are set to attempt to detect cracks as they become critical.

Crack growth – fracture mechanics

Crack growth – fracture mechanics

Implications of the assessment Design Curve has been used to set inspection intervals – allows for a safety factor over the mean curve

Crack detection There is a chance inspections may not detect cracks particularly in difficult to access locations. Reliability of detection poor without paint removal Crack Depth mm

Redundancy Redundancy is characterised by alternative load path or residual capacity to take overload Counterweight boom – critical load for bracing expected to be horizontal load from collision or non-permanent dynamics – some redundancy under working loads Counterweight pivot box - no redundancy under working loads Bucket-wheel Boom – no redundancy under working loads Cone – expect some redundancy under working loads

Probability of structural failure The factors involved in determining probability of a fatality per annum will include. Probability of Cracking (PC) Redundancy Factor (RF) Number of inspections in critical period (NI) Critical period as a fraction of 12 months (CPF) Probability of Non Detection of a crack (PND) Exposure factor as a fraction of 12 months (EXP)

Tolerable Risk

Tolerable Risk Upper limit of tolerable region 1 in 10 000 annual probability of fatality ALARP Upper limit of broadly acceptable region 1 in 10 000 000 annual probability of fatality

Tolerable Risk

Main Issues for fatigue management Cracking found by inspection not correlating with fatigue analysis (possible contributors: - fabrication defects, corrosion, dynamic effects) Amount of inspection required (cost & downtime) Exposure of personnel

Fatigue Risk Management Probability of Cracking (PC) – can’t change Redundancy Factor (RF) – some effect on counterweight and cone, not on boom or cw pivot – can make an allowance but can’t change Number of inspections in critical period (NI) – can change Critical period (CPF) – stack only Probability of Non Detection of a crack (PND) – remove paint, good access Exposure factor as a fraction of 12 months (EXP) – reduce exposure of personnel